Difficulties in achieving precise color plane alignments have hindered development of multi-color laser printers which employ single pass color printing processes. Subimages derived from color image planes must be precisely positioned, relative to each other, or else substantial image degradation results. For example, a subimage misalignment that exceeds about 50 microns produces a detectable degradation in print quality.
A number of factors contribute to misalignment of subimages in single pass color printers. One such factor is the requirement to maintain precise alignment of the multiple imaging sources. A further factor which contributes to such misalignments is imperfections in the mechanisms which drive the media sheets through the print process. For instance, each single pass color printer employs at least three and generally four developer stations, each of which employs a developer roller and a transfer roller that, in combination, propel a media sheet through the developer station.
It is known that both developer rollers and transfer rollers may exhibit non-uniform run-outs (i.e., different degrees of out-of roundness). These run-outs will cause a media sheet to be driven at different rates through a developer station, especially since such rollers are center-driven. Further, the dimensions of such rollers are known to change as a result of temperature variations, roller handling, etc. When an in-line color printer includes more than one roller, any difference in run-out or diameter between the rollers can result in color plane image misalignment.
In pursuit of perfect registration of color plane images, printer manufacturers have attempted to build up the four color plane images on a well controlled substrate, prior to transferring the image to a media sheet. An example of such a substrate is a page-size photoconductor-coated drum wherein the four color plane images are sequentially deposited thereon. Other manufacturers have used a page size intermediate transfer medium to receive the four sequential images before transferring the full image to the media sheet. While these techniques have merit, they add considerable size and complexity to the color laser printer.
Accordingly, it is an object of this invention to provide an improved system for achieving subimage color plane alignment in a single pass, color printer.
It is another object of this invention to provide an improved system for subimage color plane alignment in a laser printer, wherein the color plane images are directly placed on the ultimate media sheet.
It is a further object of this invention to provide an improved system for achieving subimage color plane alignment in a single pass laser printer, wherein apparatus such as page-size image-receiving belts and drums are avoided.